DDS Loop Technology and Coding Format
Dataphone Digital Service (DDS) is a private line digital data point-to-point, or point-to-multipoint communication service, widely deployed in the United States of America and is described in detail in the following references (incorporated herein by reference):
1, "D4 Digital Channel Bank Family", Bell System Technical Journal, Vol. 61, Number 9, Part 3, November 1982.
2. "Digital Data System", Bell System Technical Journal, Vol. 54, Number 5, May--June 1975.
3. "Generic Requirements for the Subrate Multiplexer", Issue 1 TA-TSY-000189, Bell
Communications Research Inc., April 1986.
The data rate at which a DDS customer can obtain service are the subrates 2.4 kbps, 4.8 kbps, 9.6 kbps, 19.2 kbps, 38.4 kbps and the full rates 56 kbps, 64 kbps. The local distribution of a DDS connection uses metallic, twisted-pair cables for the full-duplex four-wire transmission path between the customer premises and the serving DDS office. A Channel Service Unit (CSU) serves to terminate the four-wire loop at the customer premises. At the DDS serving office the loop is terminated with an Office Channel Unit (OCU). The OCU encodes the incoming data signals into an 8-bit byte format that adds necessary control information and, regardless of the data service rate, builds the signal up to a rate of 64 kbps.
The DDS local loop signalling format employs Alternate Mark Inversion (AMI) encoding to convert digital data signals generated at the customer premises to an AMI line code format. In AMI, a ONE bit is transmitted as a pulse transition (polarity change) and a ZERO bit as no pulse. The digital signal on the local loop is baseband, bipolar, return-to-zero, with 50-percent duty cycle. This signal has a symbol rate equal to the data rate. A binary 1 is transmitted as a positive or negative pulse, in opposite polarity to the preceding pulse. A binary 0 is represented by the absence of a pulse. The local loop signal format is described in detail in the following reference (incorporated herein by reference):
4. "Digital Data System Requirements for the Office Channel Unit", Issue 1, TA-TSY-000083, Bell Communications Research Inc., December 1984.
As shown in FIGS. 1a and 1b, the loop signal between the CPE 10 and the OCU 12 (with non-secondary channel information) is formatted in 6 bit bytes containing six channel data bits D1-D6 for the data rates of 2.4, 4.8, 9.6, 19.2 and 38.4 kpbs (FIG. 1a), or 7 bit bytes containing seven channel data bits D1-D7 for 56 kbps (FIG. 1b). The presence of network control information is indicated by modifying the standard bipolar signal. The signal is modified by inserting a violation `V` pulse into the bit stream. This pulse has the same polarity as the immediately preceding pulse (hereinafter "the "previous" pulse), thus violating the standard format.
Unrestricted insertion of violations would produce an undesirable dc component on the local loop. Therefore, a bit period is reserved two bits prior to a violation, for insertion of a bipolar pulse or no pulse, i.e., a zero, in such a manner that successive violations alternate in polarity. Calling this inserted pulse an `X` bit; modified bipolar signals are shown in FIGS. 2a and 2b with X bit values of 0 and 1. If the number of pulses since the previous violation is odd, the X bit is a zero. If the number of pulses since the last violation is even, a pulse of opposite polarity to the previous pulse is inserted into the pulse stream as the X bit. The complete violation sequence includes a forced zero between the X and V bits; therefore, the sequence is called an XOV sequence.
A secondary channel capability has been proposed to offer a companion digital transmission channel independent of the primary channel and at a lower rate. Secondary channel capability requires that the loop signal be structured so that the primary and secondary channel information can be differentiated. As shown in FIGS. 3a and 3b, the loop signal with secondary channel information is formatted in 8 bit bytes containing six primary channel data (D) bits D1-D6 for the primary channel rates of 2.4, 4.8, 9.6, 19.2 and 38.4 kbps (FIG. 3a), or 9 bit bytes containing 7 D bits for 56 kbps primary channel data, or 9 bit bytes containing 8 D bits for 64 kbps primary channel data (FIG. 3b). Each byte contains an "F" bit for framing, and a "C" bit arising out of the substitution of the secondary channel information on no more than one out of every three C bits (FIG. 3c). Intentional Bipolar Violations (BPV) in the loop signal with secondary channel information are not required. BPVs are used in the basic DDS service to transmit control and supervisory information.
MJU Circuits and Streaming Branches
The DDS provides full duplex, synchronous, end-to-end digital transmission on dedicated private line two point and multipoint circuits. A two point circuit connects two customer stations. A multipoint circuit allows several customer stations to share a common communication channel using Multipoint Junction Units (MJU) located in DDS offices. The customers control station broadcasts downstream to one or more remote stations. In the upstream direction the MJUs combine the bit streams transmitted by the remote stations into a serial bit stream for delivery to the control station. The DDS multipoint service serves only those multipoint circuits that have a single customer-control location and a number of remote stations. The duplex data path from the MJU to the customer's control station (upstream direction) is called the control channel while the duplex data paths from an MJU toward the remote stations (downstream direction) are called branches. It is the responsibility of the data customer to use an appropriate "polling technique" so that only one remote station is transmitting data toward the control station at any given time.
In the upstream direction, the MJU receives a steady stream of the Control Mode Idle (CMI) code (S1111110) or Data Mode Idle (DMI) code (S1111111) when the customer is not transmitting data on a branch. When a remote station becomes active, the idle condition will change to a data pattern provided by the customer. Functionally, the MJU can "AND" the data bits and "OR" the control bits from the branches. Note that if more than one branch is carrying an active data signal, the control channel output will be the garbled combination of the branch input signals. Since many control conditions can be signaled upstream, all network control codes (bit 8=0) received on any branch are treated as if they were the same as CMI/DMI for purposes of forming the upstream transmission byte. The MJU operation is described in detail in the following reference (incorporated herein by reference):
5. "Digital Data System--Multipoint Junction Unit Requirements", Issue 2, TA-TSY-000192, Bell Communications Research Inc., April 1986.
On a multipoint circuit, one or more idle noisy station loops (or branches), which are experiencing errors (called streaming branches), interfere with a customer's active branch. In other words, streaming branches could cause total multipoint network failures. Thus, the overall reliability of multipoint circuits worsens as the number of branches increases.
A need exits, therefore, for a system for automatically identifying noisy branches and disconnecting them from the multipoint circuit.
Proactive Maintenance
DDS is designed to be a high performance service. A typical DDS circuit terminates at the OCU at the End Office and is cross connected to T-carrier facilities for inter-office or inter-LATA haul. T-carrier uses an AMI line format and error performance of these T-carrier facilities has traditionally been monitored using BPVs. As the error performance of these facilities begins to degrade, preventive maintenance is triggered, often before the resulting performance is unacceptable given DDS performance requirements. If enough DDS circuits are present in the carrier, automatic protection switching is employed on the facility.
Unfortunately, DDS local loops are the most vulnerable part of the circuit. Since, for many of the DDS rates, BPVs are deliberately employed to transport control conditions over the local loop, heretofore it has not been possible to employ similar techniques as T-carrier for these loops.
Service capability, equivalent to DDS, multiplexed with other services (like voice frequency (VF) services) can be provided using T1 facilities directly to the customer's premises, or by local bypass service providers (principally Inter Exchange Carriers (IXCs)). This, of course, results in revenue loss for the telephone company, loss of account control, and does nothing to solve the problem for the small service sites where there are no other service needs to multiplex with the data communication needs. To further differentiate their service, IXC's employ an ESF format over the access T-carrier facility. In ESF format, part of the framing bit bandwidth carries Cyclic Redundancy Check (CRC) information which allows measurement of Bit Error Rate (BER). In the framing bit bandwidth, ESF also carries a full duplex Facilities Data Link (FDL) which, amongst other information, is used to relay back incoming BER information on the outgoing carrier facility at the customer's premise. Thus, bi-directional performance information is available at the central office to trigger an appropriate proactive maintenance strategy. The ESF framing format is described in detail in the following reference (incorporated herein by reference):
6. "Carrier to Customer Installation--DS1 Metallic Interface" ANSI T1, 403-1989.
A need exists, therefore, for an equivalent proactive maintenance system for DDS local loops.